The U.S. State Department said that the verification of any denuclearization agreement will be a difficult process.

According to Voice of America on Saturday, a department official said the verification process will likely include an important role for the International Atomic Energy Agency (IAEA) given its globally recognized status as an independent, international nuclear verifier.

The official also said the U.S. shares the IAEA’s view that North Korea’s nuclear activities are cause for grave concern.

The remark came in response to a recent report by the nuclear watchdog's Director General Yukiya Amano indicating some movements observed at North Korea's Yongbyon nuclear facility.

During an IAEA board meeting on Thursday, Amano said the movements were detected since the agency's last report made in August.

He said that at the light water reactor, IAEA observed "activities consistent with the fabrication of reactor components and the possible transfer of these components into the reactor building."

He added the reactor was likely shut down while the activities were being carried out at the facility, but it was difficult to ascertain the exact nature of the work without access to the site.